feat: Add IR for expressions.

[ritchie46]

First steps in transforming the ALogicalPlan in a high level IR. This IR will have all names properly resolved and all expressions expanded.

[codecov]

Codecov Report

Attention: Patch coverage is 95.87799% with 50 lines in your changes are missing coverage. Please review.

Project coverage is 81.29%. Comparing base (0b65c33) to head (d89a79a).
Report is 48 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/logical_plan/expr_ir.rs	92.98%	8 Missing ⚠️
crates/polars-plan/src/logical_plan/alp.rs	78.57%	6 Missing ⚠️
...y/src/physical_plan/executors/projection_simple.rs	77.27%	5 Missing ⚠️
...logical_plan/optimizer/predicate_pushdown/utils.rs	90.56%	5 Missing ⚠️
crates/polars-pipe/src/pipeline/convert.rs	92.00%	4 Missing ⚠️
...c/logical_plan/optimizer/predicate_pushdown/mod.rs	88.57%	4 Missing ⚠️
crates/polars-plan/src/logical_plan/builder_alp.rs	97.43%	3 Missing ⚠️
...rs-plan/src/logical_plan/optimizer/file_caching.rs	75.00%	3 Missing ⚠️
crates/polars-plan/src/utils.rs	93.47%	3 Missing ⚠️
.../logical_plan/optimizer/predicate_pushdown/join.rs	91.30%	2 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15168      +/-   ##
==========================================
+ Coverage   81.21%   81.29%   +0.08%     
==========================================
  Files        1346     1359      +13     
  Lines      175233   176125     +892     
  Branches     2506     2522      +16     
==========================================
+ Hits       142310   143180     +870     
- Misses      32444    32461      +17     
- Partials      479      484       +5     

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[ritchie46]

Failing test is unrelated and only on one runner.

[alexander-beedie]

Failing test is unrelated and only on one runner.

@ritchie46: FYI, I think the "unclosed socket" error(s) in tests will be fixed by #15202 🤔